Section 12.2 is the registration for the schema not the namespace, which is in Section 12.1. The URI for the schema ought to point to :schema: and not :ns:. It's in the IANA registry it just needs to be updated here.

DSKPP Client DSKPP Server
------------ ------------
E(K,R_C), AD --->
When this message is sent:
The DSKPP Client will send this message immediately following a
<KeyProvServerHello> message whose status was set to "Continue".

It should say:

DSKPP Client DSKPP Server
------------ ------------
E(K,R_C), [AD] --->
When this message is sent:
The DSKPP Client will send this message immediately following a
<KeyProvServerHello> message whose status was set to "Continue".
The AD element MUST be sent unless it was already sent in the
KeyProvClientHello message.

Notes:

The AD is carried in the <KeyProvClientHello> if sent as a result of a trigger and so is optional in the <ekyProvClientNonce>.

To handle content negotiation, HTTP requests MAY include an HTTP
Accept header field. This header field SHOULD should be identified
using the MIME type specified in Section 7.2.1. The Accept header
MAY include additional content types defined by future versions of
this protocol.

It should say:

To handle content negotiation, HTTP requests MAY include an HTTP
Accept header field. This header field SHOULD be identified
using the MIME type specified in Section 7.2.1. The Accept header
MAY include additional content types defined by future versions of
this protocol.